1. Field of the Invention
The present invention relates generally to methods of controlling a heating, ventilating and air conditioning (HVAC) system of a vehicle and, more specifically, to a method of determining windshield fogging for a HVAC system in a vehicle based on an inference from a presence of rain.
2. Description of the Related Art
It is known that a fundamental goal of a HVAC system for a vehicle is to detect and avoid windshield/window fogging conditions. In an attempt to measure and control the many variables that affect fogging, modern HVAC systems have many sensors and control actuators. A typical HVAC system might have a temperature sensor inside a cabin or occupant compartment of the vehicle, one measuring ambient temperature outside and others measuring various temperatures of the HVAC system internal workings. The occupant may have some input to the HVAC system via a set point or other adjustment. Additional sensors measuring sun heating load, humidity, etc. might be available to the HVAC system. The set of actuators might include a variable speed blower, some means for varying air temperature--including cooling and thereby extracting moisture from the air, ducting and doors to control the direction of air flow and the ratio of fresh to recirculated air.
It is known how to provide a method and control system for controlling the HVAC system to prevent windshield fogging. Such a method and control system is disclosed in U.S. Pat. No. 5,516,041 to Davis, Jr. et. al., the disclosure of which is hereby incorporated by reference. In this patent, the method and control system uses fuzzy logic to prevent windshield fogging. The control system responds to signals generated by various climate control sensors including a humidity sensor to produce a desired vehicle air temperature and air flow to detect and avoid incipient windshield/window fog conditions. Fuzzy logic and calculations are performed based on fuzzy rules and membership functions to provide nonlinear compensation.
Although the above process has worked well, it is desirable to provide a method for determining the appropriate climate control system action s to take to avoid windshield fogging or misting given the presence of rain. It is also desirable to provide a strategy of windshield de-mist based on an inference from the presence of rain. I t is further desirable to provide an auto-demist strategy based on cabin humidity sensing without using a humidity sensor. Therefore, there is a need in the art to provide a method that accomplishes these desires.